1. Field of the Invention
The invention relates to a method and arrangement for embedding a watermark in an image. The invention also relates to a method and arrangement for detecting a watermark in an image. The invention further relates to a method and arrangement for transmitting and receiving an image containing a watermark. Furthermore, the invention relates to a method and arrangement for recording an image containing a watermark onto a storage medium, and a method and arrangement for playing back an image containing a watermark from a storage medium. The expression xe2x80x9cimagexe2x80x9d is understood to mean still images as well as video.
Digital watermarking is a technique used to certify the ownership of digital image or video contents. By imperceivably hiding such ownership information into digital content, it becomes possible to prevent piracy and illegal use of this content. Typical applications of watermarking include copy-protection of digital video and broadcast monitoring.
2. Description of the Related Art
Most presently used techniques for watermarking digital images boil down to adding a pseudo-noise watermark pattern to an image. This addition may take place in the pixel domain or the frequency domain. Effectively, it means that luminance and/or color components of the watermarked image are slightly different from the original. The detection of a watermark is done by correlating the suspect image with the watermark pattern to be detected. Such a prior art method is disclosed, inter alia, in International Patent Application WO-A-98/03014, corresponding to U.S. Pat. No. 5,933,798.
A watermarking method must fulfill a number of basic requirements. Adding the watermark pattern may not affect the visual appearance of the image. There should be many different watermark patterns, since every copyright holder should have at least one distinct watermark. Detection of the watermark must be possible without using the original image. The detection must be fast so as to be able to detect watermarks in video, or to scan the World Wide Web for watermarked images. It should not be easy for attackers to remove the watermark. The method must further be robust to transformations or operations that can be applied to images or video, such as, color transformations, geometric transformations, such as, scaling, translation or rotation, data compression, such as, JPEG/MPEG, noise, embedding of other watermarks, gray scaling, image smoothing, dedicated transformations, A/D and D/A conversions, PAL/NTSC conversions, etc.
The above requirements are conflicting, and there are no methods known that fully satisfy all requirements. Many methods can easily be hacked by attackers who have a number of watermarked images at their disposal. Since detection is based on correlation (which is a linear operation), an attacker needs only a good estimate of the watermark, and he can use this to subtract the guessed watermark from images in order to mislead detection. Simple geometric transformations such as rotations or scaling can only be dealt with at the expense of considerable computing times during detection. It is also difficult to ensure sufficient variation of watermarks. The number of bits that can be embedded using one correlation is limited.
It is an object of the invention to provide a method and arrangement for embedding a watermark which alleviates the above mentioned drawbacks.
To this end, the invention provides a method of embedding a watermark in an image, comprising the steps of identifying a given number of most significant image pixels; defining a watermark in the form of a pattern of pixels; and locally warping the image so as to move a significant percentage of the most significant image pixels to the vicinity of the watermark pattern.
The corresponding method of detecting the watermark comprises the steps of identifying a given number of most significant image pixels; determining the distances of said most significant image pixels to the watermark pattern; and generating a detection signal indicating that said predetermined watermark is embedded in the suspect image, if the average distance of said most significant pixels to the watermark pattern is significantly smaller than the average distance of all image pixels to the watermark pattern.
The watermark is thus embedded by slightly changing geometric features of the image instead of changing luminance or color components. This makes removal of the watermark a complex task because it requires image dependent processing, in contrast with simply subtracting an estimated watermark from the image, which works for all images in most current methods. The method is also more robust against geometric transformations such as scaling and rotations, because detection can be done in a shorter time. Further, the method allows a sufficient variation of watermark patterns to be used. Checking a suspect image against many different watermark patterns is computationally a drastic improvement over current methods.